Arbuscular mycorrhizal symbiosis (which links a plant to a fungus) thus gives plants a mechanism for improving their supply of water and mineral nutrition. This association has been in existence for 400 million years and appears to have accompanied plants in their colonization of the terrestrial environment.
At present it involves about 80% of plant species. In a more recent era, about 60 million years B. P., the symbiosis which became established between soil bacteria, Rhizobium species, and leguminous plants doted them with the ability, unique among mass-produced crop plants, to capture nutrient nitrogen from the air. Rhizobium forms specialized organs, nodules, on the plant roots. These are capable of transforming atmospheric nitrogen into ammonium that can be directly assimilated by the plant. In return, the plant supplies the microorganisms with nutrients in the form of complex carbohydrates.
Scientists have for many years been seeking to unravel the genetic mechanisms that govern such mutually beneficial relationships, on the one hand between plants and bacteria, on the other between plants and fungi. Investigations by a French team in 2000 had shown that some genetic signalling mechanisms operating in the symbiosis between leguminous plants and Rhizobium type bacteria and such plants and mycorrhizal fungi involved a common genetic element named SymRK. This type of gene was already known to operate in the recognition of Nod factors, signalling substances emitted by the Rhizobium type bacteria which are essential for root nodule formation.
The actinorhizal plants make up another category of plants which have acquired the ability to live symbiotically with a nitrogen fixing bacterium, in this case Frankia. These pioneer plant species, whose host-symbiont mechanisms remain little studied, generally colonize disturbed environments, such as volcanic soils or mining-affected ground, and nitrogen-poor terrains such as moraines or sandy soils. About 260 species of actinorhizal plants exist, spread among 24 genera and classified into eight families of angiosperms, flowering plants. An IRD team, jointly with a laboratory of the University of Munich, turned particular attention to the tropical tree Casuarina, or Australian pine. The first step employed molecular methods to find the sequence coding for the SymRK gene in the Casuarina genome. Once isolated, the question was whether or not Casuarina needed this gene to establish its symbiosis with the bacterium Frankia.
The team therefore developed transgenic plants in which SymRK gene expression was strongly reduced. Subsequent comparison of these plants’ ability to form symbiotic root nodules with that of control plants showed that the plants with lowered SymRK gene expression produced only half as many root nodules as the controls. The same modified individuals also showed strongly reduced mycorrhization compared with the unaltered Australian pine. The results therefore demonstrated that the weakened SymRK gene expression produced a considerable loss of Casuarina’s nitrogen-fixing ability and also a reduction in its aptitude to form mycorrhiza. More generally, these conclusions bring out the fact that, in nitrogen fixing plants, a common genetic factor seems essential for setting-up the three types of symbiotic association involving bacteria (Rhizobium or Frankia) or a mycorrhizal fungus.
Improved understanding of these genetic mechanisms could in the coming years contribute to the development of procedures for performing the transfer of the genetic material necessary for atmospheric nitrogen fixation to plants like cereals, which do not possess this faculty. Although rice, for example, establishes a symbiotic relation with a mycorrhizal fungus, it is incapable of developing nitrogen fixing nodules.
Modification of its genome to equip it with this ability could then open the way to considerable reduction of input of nitrogen fertilizers on this crop and thus cut down the resulting soil pollution.
Grégory Fléchet | alfa
Researchers discover natural product that could lead to new class of commercial herbicide
16.07.2018 | UCLA Samueli School of Engineering
Advance warning system via cell phone app: Avoiding extreme weather damage in agriculture
12.07.2018 | Leibniz-Zentrum für Agrarlandschaftsforschung (ZALF) e.V.
A new manufacturing technique uses a process similar to newspaper printing to form smoother and more flexible metals for making ultrafast electronic devices.
The low-cost process, developed by Purdue University researchers, combines tools already used in industry for manufacturing metals on a large scale, but uses...
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
23.07.2018 | Science Education
23.07.2018 | Health and Medicine
23.07.2018 | Life Sciences